This invention relates to inks for ink jet printers, more particularly, for thermal ink jet printers. The invention further relates to an ink jet recording process.
In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in U.S. Pat. No. 4,463,359 to Ayata et al. At one end, these channels communicate with a relatively small ink supply chamber. At the opposite end, the channels have an opening referred to as a nozzle. A thermal energy generator, for example a resistor, is located in each of the channels a predetermined distance from the nozzles. The resistors are individually addressed with a current pulse to momentarily vaporize ink in the respective channels and thereby form a bubble. As a bubble grows, the ink bulges from the nozzle, but it is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and the bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle, resulting in the separation of the bulging ink as an ink droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides momentum and velocity to the droplet in a substantially straight line direction towards a recording medium, such as paper.
Water is generally thought to be an ideal jetting material. It forms bubbles with the largest possible volume expansion upon vaporization. However, although approximately a third of the heat from the heater is deposited into the ink, only a small portion of this heat is actually used to form a bubble. The rest of the heat is conducted too far into the ink to be used.
As the bubble is forming, it is nucleated in a thin layer above the heater. Its growth is fed initially by the high pressure in the newly formed bubble and, in addition, by the transfer of heat and evaporation of ink from the liquid-gas interface. Ultimately, the ink cools below the liquid boiling point and bubble expansion continues due to momentum alone.
Although water is ideal for forming the bubble, once the image has been formed on the recording medium, the image tends to smear since water tends to remain on the surface of the medium and is not curable. Therefore, it would be desirable to form an ink that can form a sufficient bubble but at the same time can be cured once the image has been formed on the recording medium.
In addition, water tends to cause cockle and curl in the recording medium. Therefore it would be desirable to form an ink that will reduce or eliminate the cockle and curl of the recording medium that results from known water-based inks.
Curable inks are known in the printing industry. For example, U.S. Pat. No. 4,680,368 to Nakamoto et al. discloses an ultraviolet curable ink composition comprising a polyurethane polymethacrylate obtained by reacting a polyisocyanate compound of the formula: ##STR1## wherein R.sub.1 is a hydrogen atom or a methyl group, and n is an integer of from 1 to 20, with a hydroxyl group containing methacrylate and having in one molecule at least two methacryloyl groups and at least two urethane bonds, a radical polymerizable low molecular weight compound, and a photopolymerization initiator.
In addition, U.S. Pat. No. 4,443,495 to Morgan et al. discloses a heat curable conductive ink that comprises (1) an ethylenically unsaturated member of the group consisting of (a) a liquid ethylenically unsaturated monomer or oligomer of the formula: ##STR2## wherein R is H or CH.sub.3, R.sub.1 is an organic moiety and n is at least 2, (b) a polythiol in combination with (a), (c) a polythiol in combination with a liquid ethylenically unsaturated monomer or oligomer of the formula: ##STR3## wherein R.sub.2 is H or CH.sub.3, R.sub.3 is an organic moiety and n is at least 2, and (d) mixtures of (a), (b), and (c); (2) a thermal initiator; and (3) an electrically conductive material. Heating of the composition in a desired pattern on a substrate results in a printed electric circuit.
Further, U.S. Pat. No. 4,751,102 to Adair et al. discloses a radiation curable ink composition comprising pigment and a photohardenable composition. The photohardenable composition comprises a free radical polymerizable or cross-linkable compound and an ionic dye reactive counter ion compound that is capable of absorbing actinic radiation and producing free radicals that initiate free radical polymerization or cross-linking of the polymerizable or cross-linkable compounds.
U.S. Pat. No. 4,334,970 to Lombardi et al. discloses a photosensitive resin system that is essentially solvent-free and contains an ester produced from an unsaturated organic acid and a polyhydroxyl containing material, a photoinitiator, a carbonyl initiator, a monomer capable of reacting with an acrylic monomer, and an unsaturated hydroxyl containing polymer hydrocarbon.
U.S. Pat. No. 5,395,724 to Morrison et al. discloses a liquid developer containing a curable liquid vehicle including epoxies, vinyl ethers, styrenes, indenes, vinyl acetals, ketene acetals, aliphatic .alpha.-olefins and/or mixtures thereof. The reference does not teach ink jet inks or an ink jet printing process. In addition, the reference does not teach a composition comprising ethylene glycol monovinyl ether. Further, the reference does not teach adding water to the curable liquid vehicle. In fact, adding water to the developer would cause the developer to be conductive and thereby interfere with its function as a developer.
One potential difficulty with the use of curable inks containing a photoinitiator is that the ink may be subject to premature polymerization. For example, if enough stray light enters the nozzles of the printhead, polymerization can be initiated, destroying the ink and potentially destroying the printhead. One possible solution is to increase the tolerance of the ink to stray light, however, this increases the delay in the onset of polymerization even after printing, potentially reducing the achievable image quality. In addition, because of high printer through-put or because of rapid wicking of the ink along paper fibers, rapid curing of the ink may be required. Although the speed of curing may be increased by using a high intensity light source, which provides rapid curing, such a light source also generates a considerable amount of ozone, requiring safety precautions that may be unacceptable in some products.